1. Field of the Invention
The present invention relates to a developing device and a developing method, and in particular to an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer, and a developing device and a developing method that are applied to such an image forming apparatus.
2. Description of the Related Art
Conventionally, as an electrophotographic method, there is a method of utilizing a photoconductive material to form an electric latent image on a photosensitive drum by various ways, subsequently developing the latent image with toner to visualize it, transferring a toner image on a transfer material such as a sheet depending on a situation, and then fixing the toner image by heat, pressure or the like to obtain a copy.
On the other hand, since improvement of image resolution, definition, and the like is currently required, development of a method of forming a thin layer of toner and an apparatus for the method is essential, and several measures have been proposed to fulfill the requirement.
In recent years, a contact single component developing method has been proposed, which performs development with a configuration for pressing against a surface layer of a photosensitive drum using a semiconductive developing roller or a developing roller having a dielectric layer formed on its surface.
Here, a schematic sectional view of a conventional image forming apparatus using a DC contact single component developing method is shown in FIG. 17. In this method, a photosensitive drum 61 and a developing roller 65 abut each other. First, the photosensitive drum 61 is charged by a charging roller 62, and a latent image is formed on the photosensitive drum 61 by a laser beam from an exposing unit 63.
Then, the latent image is visualized by a developing device 64. Thereafter, toner 68 on the developed image is transferred to a transfer material 13 by a transferring roller 9. The toner 68 that was not transferred and remains on the photosensitive drum 61 is scraped off by a cleaning blade 10 and contained in a cleaning container 69.
This developing device 64 has an elastic roller 66 that is in pressurized contact with the developing roller 65 at a position on the upstream side in the rotating direction of the developing roller 65 from an elastic blade 67 inside a developer container 60 containing nonmagnetic toner 68 as a single component developer. The elastic roller 66 rotates in the direction of an arrow D to supply the toner 68 on the developing roller 65. The toner 68 supplied on the developing roller 65 is conveyed in accordance with the rotation of the developing roller 65 and is given a charge by friction at the abutment part of the elastic blade 67 and the developing roller 65 to form a thin layer.
Then, the toner 68 forming the thin layer is conveyed by the developing roller 65 and is supplied for development of an electrostatic latent image at the abutment part with the photosensitive drum 61. Thereafter, the toner 68 that was not used for development in the abutment part of the photosensitive drum 61 and the developing roller 65 and remains on the developing roller 65 is scraped off by the elastic roller 66.
In addition, new toner 68 is supplied to the developing roller 65 by the elastic roller 66 as described above, and the above-mentioned actions are repeated. In addition, the above-mentioned single component developer consists of toner prepared by internally or externally adding an auxiliary agent to base resin according to necessity. An auxiliary agent to be internally added is represented by a charging polarity agent, and an auxiliary agent to be externally added is represented by a plasticizer.
Further, in the developing method using the DC contact single component developing method, it is necessary to convey sufficiently charged toner to a developing region and visualize a latent image by such toner in order to form a visualized image with high quality of a predetermined density.
However, it is extremely difficult to convey sufficiently charged toner to a developing region over a long period. This is because triboelectric property is lowered due to deterication of a developing roller, an elastic blade and an elastic roller resulting from a multiplicity of times of frictions, and toner is deteriorated due to separation of externally added agent.
Therefore, in recent years, a method of using means for electrically charging toner using a toner charging roller for the purpose of high triboelectric stability and fog reduction has been proposed as disclosed in Japanese Patent Application Laid-open No. Hei 11-119546 and Japanese Patent Application Laid-open No. Hei 11-119547.
Here, a developing method using a toner charging roller is shown in FIG. 18. FIG. 18 is a schematic sectional view of a developing device of a contact developing method using a conventional toner charging roller. This developing device is an apparatus of the above-mentioned method to which a toner charging roller 72 is attached. The developing device has an elastic roller 76 that is in pressurized contact with a developing roller 75 at a position on the upstream side of the rotating direction of the developing roller 75 from an elastic blade 73 inside a developer container 70 containing nonmagnetic toner 74 as a single component developer. The elastic roller 76 rotates in the direction of an arrow D to supply the toner 74 on the developing roller 75.
The toner 74 supplied on this developing roller 75 is conveyed in accordance with the rotation of the developing roller 75 and is given charge by friction at the abutment part of the elastic blade 73 and the developing roller 75 to form a thorn layer.
Moreover, the toner 74 is given charge electrically by discharge of the toner charging roller 72. Then, the toner 74 to which charge is given is conveyed by the developing roller 75 and is supplied or development of an electrostatic latent image at the abutment part with a photosensitive drum 71.
Thereafter, the toner 74 that was not used for development at the abutment part of the photosensitive drum 71 and the developing roller 75 and remains on the developing roller 75 is scraped off by the elastic roller 76. In addition, new toner 74 is supplied to the developing roller 75 by the elastic roller 76 as described above, and the above-mentioned actions are repeated.
If the above-mentioned toner charging roller is used, toner can be conveyed to a developing region of sufficiently charged toner even it an amount of toner charge (hereinafter referred to as a toner charge amount) decreases due to use over a long period, and an image defect such as a fog can be prevented.
However, even if a toner charging roller is used, a toner charge amount changes over time due to separation of an externally added agent of toner, etc. As a result, there is a problem in that an image quality varies in such a case in which image densities at an initial time of use and after a long term use are different.
The present invention has been devised in view of the above and other drawbacks, and it is an object of the present invention to provide a developing device and a developing method that are capable of steadily supplying sufficiently charged toner over a long period of time and always giving a high image quality.
In order to solve the above problem, a developing device in accordance with the present invention comprises: a developer bearing member for bearing and conveying a developer in order to apply the developer to an image bearing member, wherein a first voltage is applied to the developer bearing member; further comprising a developer charging member for charging the developer born by the developer bearing member, a second voltage is applied to the developer charging member and controlling means for variably controlling a potential difference between the first voltage and the second voltage are provided.
In this case, in the present invention, the developer bearing member and the developer charging member may be formed in a roller shape, for example, the developer bearing member may be configured of a developing roller, or the developer charging member may be configured of a toner charging roller.
Further, in the present invention, the controlling means controls the potential difference according to information concerning a use state of the developing device. In this case, a developing device wherein the information concerning a use state or the developing device, may be a number of times of image forming operations applied to the image bearing member by the developing device. Further, according to the present invention, an image is formed on a recording material using the image bearing member, and the information concerning a use state of the developing device may be a number of recording materials on which an image is formed. Further, the information concerning a use state of the developing device may be a number of rotations of the developer bearing member. Further, the information concerning a use state of the developing device may be a duration of applying the first voltage.
Further, according to the present invention, the developing device has a storage medium for storing the information.
Further, the developing device is provided in a cartridge that is detachably attachable to the main body of an image forming apparatus, and the cartridge has the storage medium for storing the information.
Further, according to the present invention, the potential difference in forming an image is determined according to densities of a plurality of pattern images that are formed using the developing device by changing the potential difference. Further, according to the present invention, the developing device has density detecting means for detecting the densities of the pattern images.
Further, according to the present invention, the developing device and the image bearing member are provided in an image forming apparatus, and the image forming apparatus has density detecting means for detecting the densities of the pattern images.
Further, according to the present invention, the cartridge is provided with the image bearing member, and the developing device may be provided in a process cartridge, which is detachably attachable to a main body of an image forming apparatus, together with the image bearing member. Further, the developing device and the image bearing member may be provided in an image forming apparatus.
In addition, in the present invention, a developing method is provided which applies a first voltage to a developer bearing member that conveys a developer in order to apply the developer to an image bearing member, applies a second voltage to a developer charging member that charges the developer borne by the developer bearing member and variably controls a potential difference between the first and second voltage.
Therefore, according to the present invention, the controlling means for controlling a potential difference between a first voltage applied to an image bearing member and a second voltage applied to a developer charging member is provided. Thus, even if conditions for forming an image are changed, a sufficiently charged developer can be provided steadily.
In addition, a potential difference between a first voltage and a second voltage is controlled based on a use state of the image forming apparatus such as the number of times of image forming operations executed by the developing device. Thus, a developer with an appropriate charge state can be provided according to a change accompanying the use of the image forming apparatus.
In addition, the developing conditions controlling means defines a potential difference between a first voltage applied to the developer bearing member and a second voltage applied to the developer charging member in forming an image based on the density data read by the density detecting means. Thus, an appropriate potential difference between the first voltage and the second voltage can be determined according to a state of the image forming apparatus
In addition, convenience of a user can be improved because the developing device and the image bearing member are provided in the process cartridge detachably attachable to the image forming apparatus.